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[TG] [TG11] ST50 Depth (and Speed?) to T121 Hull transmitter
05-29-20, 11:31 PM (This post was last modified: 06-09-20 10:34 PM by Tom - Raymarine - Moderator.)
Post: #1
[TG11] ST50 Depth (and Speed?) to T121 Hull transmitter

I have a 1995 Catalina 320 with original Autohelm ST50 depth and speed displays and transducers. I installed a Tacktick system for wind with T120 wind transmitter, T111 and T112 displays, and a T122 NMEA interface connected to a bridge to the Seatalk signals from the Autohelm speed and depth network. Everything works fine and I can see wind information on the Seatalk network and speed and depth on the wireless displays.

I would like to eliminate the old Autohelm displays but continue to use the transducers by connected to a T121 hull transmitter. According to this thread,, the depth transducer should be compatible. However, today I tried hooking it up to the hull transmitter and the depth was being reported as random values nowhere close to the actual depth (depth was about 16' and the values were in the range 60-180').

Before I spend too much time trying to get this to work, please can you confirm that it should work? Also, should the original speed sensor be compatible with the T121?

Thanks for your help.

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06-09-20, 11:34 PM
Post: #2
RE: [TG11] ST50 Depth (and Speed?) to T121 Hull transmitter
Hello adrianba,

Tacktick/Wireless-system speed transducers aren't cross-compatible with ST50/60-type ones.
On the depth data: does the same transducer work with the ST50s? Wireless Hull Transmitters are not a powerful depth sounder output (power consumption is minimised in order to be able to run off solar on battery-less boats) and possibly with an aging transducer and soft, muddy bottom it's struggling to pick up a decent bottom echo. Unfortunately there's very little in the way of diagnosis with this that you can do apart from trying another transducer hung over the side.


Raymarine since 1999.
Interests: Diagnosis of problems in sonar/fishfinders, NMEA2000, ethernet comms, autopilots, thermal cameras
Location: Sydney, Australia.

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